1. Field of the Invention
The present invention relates to an image signal processing device for processing an image signal.
2. Description of the Related Art
A color encoder for modulating base-band color-difference signals into a chrominance signal has heretofore been known as one example of an image signal processing device for processing an image signal.
Such a color encoder has a circuit arrangement, for example, such as that shown in FIG. 1. In the shown circuit arrangement, burst flags (R-Y.BF and B-Y.BF) are added to base-band color-difference signals (R-Y and B-Y in FIG. 1) by burst adding circuits 41 and 42, respectively, and the outputs of the burst adding circuits 41 and 42 are respectively clamped by clamping circuits 43 and 44. The outputs of the respective clamping circuits 43 and 44 are balanced-modulated in balanced modulators 45 and 46 by using subcarrier signals SC90 and SC which are 90.degree. out of phase with each other, and the outputs of the balanced modulators 45 and 46 are added together in an adder 47, whereby a chrominance signal to which the burst signals have been added is formed from the base-band color-difference signals.
Another circuit arrangement of the color encoder is shown in FIG. 2. In the shown circuit arrangement, the base-band color-difference signals (R-Y and B-Y in FIG. 2) are clamped by clamping circuits 51 and 52, respectively, and the outputs of the respective clamping circuits 51 and 52 are balanced-modulated in balanced modulators 53 and 54 by using the subcarrier signals SC90 and SC which are 90.degree. out of phase with each other. Then, the outputs of the balanced modulators 53 and 54 are added together in an adder 55, whereby the base-band color-difference signals are formed into a chrominance signal. Meanwhile, a burst flag BF is subjected to quadrature two-phase modulation by a quadrature two-phase modulator 56 separately from the base-band color-difference signals, thereby forming a burst signal. The burst signal is added to the chrominance signal outputted from the adder 55 in an adder 57.
However, either of the above-described conventional circuits has a number of disadvantages. For example, either circuit needs a special circuit block for adding burst signals to a chrominance signal. In the circuit shown in FIG. 1, when the burst flags are to be added to the respective base-band color-difference signals, it is necessary to adjust the DC levels of the burst flags to those of the color-difference signals or to adjust the offset levels of the color-difference signals and the burst flags. In the circuit shown in FIG. 2, it is necessary to form different burst signals to be added to a chrominance signal, in accordance with whether a chrominance signal to be formed conforms to the NTSC television system or the PAL television system. Particularly, if a chrominance signal conforming to the PAL television system is to be formed, it is necessary to switch the phase of the burst signal, so that a circuit having a complicated arrangement is needed and it is, therefore, difficult to adjust the circuit.